Pulsed PECVD for Low-temperature Growth of Vertically Aligned Carbon Nanotubes†

Pulsed plasma-enhanced (PE)CVD is used for the growth of vertically aligned multiwall carbon nanotubes (CNTs) at a low temperature range of 350–490 °C. A pulsed plasma is generated by the application of a rectangular negative pulse to the substrate electrode with an on time of 4.5 μs, off time of 5.5 μs, duty cycle of 45%, and pulse repetition frequency of 100 kHz. CNTs are synthesized from Ni catalyst film of 20–40 nm thickness deposited on a silicon substrate under pressures of 0.01 and 0.5 Torr by magnetron sputtering. The effect of Ni catalyst film morphology on low temperature growth of CNTs by pulsed PECVD is studied. It is found that CNTs grown from Ni catalyst films depend on the process pressure employed to prepare the film by magnetron sputtering. A comparison with the direct current (DC) discharge-produced CNTs reveals that the growth rate of pulsed plasma-produced CNTs is two times higher. CH species density is studied using optical emission spectroscopy (OES) by an actinometrical approach, which shows that DC discharge plasma has a higher CH concentration, but still has a lower growth rate. Further, it is observed that using pulsed plasma, growth of CNTs is possible at temperatures down to 350 °C, whereas in the case of DC discharge plasma, CNTs growth is possible only at temperatures down to 450 °C in the present experimental set-up. Possible reasons for the better performance of pulsed plasma in respect of growth rate and low temperature growth are discussed.